1. Field of the Invention
This invention relates to a method of quantizing image signals carrying gradation, and an apparatus for carrying out the method. This invention particularly relates to a method of quantizing image signals, which is suitable for the case where image signals are to be quantized coarsely, and an apparatus for carrying out the method.
2. Description of the Prior Art
In recent years, it has become a common practice to store image signals on a recording medium such as an optical disk, and read from the recording medium when necessary for reproducing a visible image on a cathode ray tube (CRT) or the like. It is also common to transmit image signals by facsimile or the like, and to reproduce a visible image on the signal receiving side based on the received image signals.
In the case where image signals are stored or transmitted, it is desirable that the amount of the image signals be decreased in advance. This is because, in the case of image signals carrying gradation, the amount of the image signals is generally very large, and the signal storage cost, the signal transmission cost, or the like becomes markedly high when the image signals are directly stored or transmitted.
As one of the methods for decreasing the amount of the image signals, there has heretofore been known a method of compressing the image signals by redundancy suppression encoding processing such as prediction encoding processing or encoding by orthogonal transformation. Besides, there may be used a method of decreasing the density resolution of the image signals, i.e. a method of making the quantization of the image signals coarse, or a method of combining the redundancy suppression encoding processing with the processing for decreasing the density resolution.
However, when the density resolution is decreased by making the quantization of the image signals coarse as mentioned above, an artifact such as an artificial contour readily arises in the reproduced visible image, though it is possible to decrease the amount of the image signals.